Rotary multitubular kiln for roasting molybdenite and similar sulfides

ABSTRACT

Kiln for roasting sulfide ores comprising a cage of hollow oreholding tubes mounted to rotate in a reverberatory furnace about a horizontal axis, said tubes being open at one end for the introduction of ore and vented at the other end for the release of gas generated by the roasting ore.

United States Patent [72] Inventors Carlos Arrizaga Alameda Bernardo OHiggins 1146, Office 406; Anibal Gajardo, Sebastian del Piombo 7691, Santiago, Chile [2]] Appl. No. 760,273

[22] Filed Sept. 17, 1968 [45] Patented Jan. 12, 1971 [32] Priority Sept. 18, 1967 [33] Great Britain [3 l No. 42,468

[54] ROTARY MULTITUBULAR KILN FOR ROASTING MOLYBDENITE AND SIMILAR SULFIDES 12 Claims, 3 Drawing Figs.

52 us. Cl

[51] Int. Cl F27b 7/08 [50] Field ofSearch 263/34, 18; 34/108; 23/279 [56] References Cited FOREIGN PATENTS 860,625 9/1940 France 263/34 Primary Examiner-John J. Camby Attorney-Holcombe, Wetherill & Brisebois ABSTRACT: Kiln for roasting sulfide ores comprising a cage of hollow ore-holding tubes mounted to rotate in a reverberatory furnace about a horizontal axis, said tubes being open at one end for the introduction of ore and vented at the other end for the release of gas generated by the roasting ore.

Bis-54,510

PATENTEU JAN 1 219m SHEH 2 BF 3 N of.

ROTARY MULTITUBULAR KILN FOR ROASTINC MOLYBDENITE AND SIMILAR SULFIDES The present invention relates to the roasting of sulfide ores to obtain their oxides. in particular, it refers to a rotary multitubular kiln for roasting molybdenite and similar sulfides.

Stationary kilns for the treatment of molybdenite are known, in which said material in a powdered state is stirred by rakes or similar means in order that the material be successively brought into contact with air and roasted at the reaction temperature. Typically these kilns are built with successive floors, like the Wedge or the Nichols-Herreshof types.

Molybdenite roasting requiresa critically controlled treatment in order that the roasting may be'pr'operly carried out, for it is known that the reaction takes place only within a narrow range of temperature, i.e., between 630650 C. No reaction takes place below 600 C., and over 650 Chagglomeration appears in the material, inhibiting roasting. Further, for the correct operation of such internally fired kilns, i.e., kilns in which the products of combustion of the fuel come into contact with the molybdenite being roasted, and in order to avoid contamination of the molybdenite with undesirable products such as sulfur, or other impurities, the quality of the combustion gasmust be carefully chosen and, for this reason, preference is given to the use of natural or bottled gas, the latter being rather expensive. lf fuel oil is used, it must be chosen from among those oils which are as free as possible from deleterious impurities which could contaminate the molybdenum trioxide in production.

On the other. hand, kilns of the above-mentioned type are large and may have a great output capacity which may reach to 250 or 300' tons permonth of molybdenum oxides. When such quantities of molybdenite are-dealt within a single continuous kiln, the exothermic roasting reaction is made use of (i.e., when the molybdenite is burned), in order to maintain combustion and economize on liquid or gaseous fuel; but this implies a serious complication for, when sulfur burns, it gives off additional heat that could well take the kilns internal temperature above the optimum reaction temperature, requiring the use of additional cooling means, generally water,,in order to keep the kiln temperature the proper range, and this I The finely powdered molybdenite is fluidized by a stream of air which produces a molybdenite suspension within a vertical tubular kiln. This kiln is heated until combustion of the sulfur in the molybdenite takes place. The'reaction'temperature is controlled by cooling means. The method has also the drawis externally heated. A group of such enclosures may be rotated within a reverbatory type of furnace, and-it thus becomes feasible to control the reaction temperature with ease, and therefore the quality of the product obtained. The process thus acquires the nature of a semicontinuous treatment which presents many advantages, as will be shown later.

Therefore, it is a main object of the present invention to provide a rotary kiln formed by, individual enclosures, in which molybdenite or similar sulfides are roasted by merely contacting them with air, whether enriched with oxygen or not, and supplying external heat to said enclosures in order that the combustion gases will not come in contact with themolybdenite being processed.

Another object of the invention is the processing of in dividual limited lots of molybdenite or similar sulfides in separate containers working jointly as 'a unit, in order that no exothermic reaction can take place to such a degree, that ignition of the sulfur of the treated molybdenite takes place. Therefore a better control of the reaction temperature is achieved. Still another object of the present invention is thegrouping together'of the several processing containers in such a way that'the discharging and chargingof each container may be successively carried out quickly and expeditiously. Thus, it is possible to obtain a type of output intermediate between discontinuous and continuous systems.

Other objects and advantages will appear as the detailed description of this invention proceeds.

Basically, the kiln of this invention is contained within a reverbatory type furnace and is in somewhat the shape of a rotary cylindrical cage, formed by tubes, which might be termed a squirrel cage." These tubes are internally supplied with air or, in some cases, oxygen enriched air, or even oxygen alone, and they are heated externally by combustion produced in the reverbatory furnace; thus the combustion gases do not come in contact, at any moment, with the material being processed.

During roasting, it is necessary that a large superficial area I of the sulfide,in a finely divided state, be exposed to the ac- On the other hand, the degree of heating of the molybdenite is fairly I critical. It is known that roasting is ineffectual under 600 C. Above 620 C., molybdenite shows a tendency to distillate and, above 650' C., agglomeration takes place. Finally, above 700 C., molybdenite sublimes. Therefore, the process temperature conveniently has to be maintained within a rather narrow range of about 630-650 C.

Hence, and also to prevent self-combustion of the molybdenite at the roasting temperature, a rotary multitubular kiln system is contemplated, as hereinbefore stated, wherein each tube does not contain more than about 1100 to 110 pounds of back that the proportion of fines produced is very high and 7 these are entrained in the air current. Their total may run up to some 30 percent of the material being treated. Such fines must be trapped by cyclones and recirculated. The com bustion gases are internally in contact with the molybdenite and properly chosen fuel is therefore required to obtain good quality molybdenum oxide. y

We have now found that when molybdenite is roasted in moderate quantities or batches of, say, some 70 to 90 pounds, the exothermic reaction is not sufficiently strong to maintain the combustion of sulfur and continuous external heating of the molybdenite is now necessary inorder that the roasting may proceed. Thisavoids the production of uncontrolled internal heat within the molybdenite proper and, on the other hand, it does awaywith the need for getting rid of this extra heat, which is one of the drawbacks of the conventional proce5es. We have also observed that separating the molybdenite from the combustion gases leadsito a far superior control of the quality of the molybdenum trioxide'obtained, for molybdenite is only in contact with atmospheric air, which may or may not be enriched with oxygen, or sometimes even with industrial oxygen, within a nearly closed enclosure which molybdenite per charge. We have found it preferable that each individual tube of the cageshould have an internal diameter in the range of 8 to l2'inches. Finely divided molybv denite, as received from producers generally in the form of flotation concentrates, is loaded into each of these tubes.

The action of ordinary or oxygen-enriched air, or of 0,, on molybdenite during roasting, is superficial. Therefore it is essential that a large superficial area 'of finely divided molybdenite be exposed to the action of said gases during roasting, while at the same time the powdery mass is heated rapidly. This is obtained in seemingly the best manner, by introducing into the tubes of the multitubular kiln according-to this invention, sufficient molybdenite to fill said tubes to a depth equivalent to about one-fifth of their diameter. The tubes conveniently have a length in the range of 2.50 -3.00 meters. For example, with tubes having an internal diameter of 250 mm. filled to one-fifth of their diameter, the ratio between the treated mass and the active surface area in contact with the air or gases, is then 3.6 cc. of material per cm of surface exposed to air or gases.

In view of the fact that the amount of molybdenite charged is on the small side, these tubes are unloaded and loaded by hand. For example, if the tubes of the kiln have an interior diameter of 250 mm. and a length of about 2,800 mm., the charge for each tube would weight about 85 I pounds and unloading and reloading can easily be done by hand within minutes. On the other hand, the processing time for molybdenite is about 8 hours, self-combustion having been eliminated.

If we now place a plurality of such tubes in a squirrel cage arrangement, we can in a small space simultaneously heat some 8 to 12 tubes, so that with continuous operation the output from such a kiln might come to about 1 ton per day, working around the clock on three shifts,the tubes being unloaded and reloaded according to a predetermined schedule at given time intervals.

Loading and unloading said tubes can be done, as will be indicated in the detailed description to follow, without stopping the rotation of the kiln cage within the furnace, if desired, but it should be noted that a 10-12 minute stopping of said cage for such loading and unloading of a particular tube does not significantly impair the action of the kiln.

The kiln is also provided with adequate means for supplying atmospheric air or gases to the roasting tubes, means for heating said tubes, means for causing rotation of the cage at a slow speed, which may be in the range of 1-1.4 rpm, and accessory means for evacuating separately the combustion gases and process gases, and for the recuperation of fines, as will be explained later on in detail in this specification, with reference to an embodiment that will be described for purposes of illustration, but it will be appreciated that the invention is not to be considered limited to the details thereof. In this description, reference will be made to the accompanying drawings, in which:

FIG. l is a vertical cross section taken through a kiln according to the invention, within a furnace, depicted diagrammatically and with the rotary multitubular kiln of the invention in its working position;

FIG. 2 is a horizontal cross section, also shown diagrammatically, of the apparatus shown in FIG. I;

FIGS. 3 and 4 show tools which may be used for loading, unloading and distributing the material in said tubes;

FIGS. 5 and 6 show the position of the material within the tubes at the moment of loading, and during rotation of the kiln, respectively; and

FIG. 7 is a schematic view showing the extractor and cyclone which may be attached to draw gas from the axle of the kiln.

Fundamentally, the apparatus according to this invention consists of two essential parts: the furnace proper, and the rotary multitubular kiln for roasting molybdenite and similar sulfides. The furnace proper is of the conventional reverberatory type which can be heated as desired; but when used in com-- bination with the rotary multitubular kiln structure of the present invention, it is said combination of furnace and kiln that forms a major part of the present invention.

Preference is given to the use of a furnace with two lateral combustion chambers, one on each side of the rotary multitubular cagelike kiln of the invention. Preferably, these, combustion chambers are provided with two burners each, said burners being preferably of the atomizing type, which use liquid fuel, such as kerosene or fuel oil, and are provided with means for controlling the amount of fuel supplied thereto, to better control .the temperature within the furnace and the uniformity of said heating. Suitable pyrometers should be installed within the furnace for observing the temperatures, either by inspection and manual control of said burners, and the signals from said pyrometers may be used for the automatic control of said burners.

Alternatively, a furnace heated by electric resistances may be used.

With four such burners, one at each corner of the furnace, the flames rise above lateral walls in the'combustion chambers, rebound on the crown-shaped roof of the furnace and descend through the zone containing the cagelike kiln. The

products of combustion then pass through lateral ports in the lower part of the furnace structure and finally exit to atmosphere through one or more chimneys or flues.

Such a furnace, adapted to the needs of the invention, is shown in FIGS. 1 and 2, in which 12 indicates the combustion chambers, 14 the burners, 11 the pyrometers, 16 the arched roof, 18 the heating chamber containing the rotary multitubular kiln, and 20 the ports for leading the products of combustion to conventional chimneysor flues (not shown),The arrows in FIG. 1 indicate the path of flame and combustion gases. .1}-

The furnace is initially built without the front wall 22, which is only built after the rotary kiln has been brought into position inside the furnace. Likewise, a small central opening 24 is provided in the rear wall of the furnace, for the passage of one end of the rotary kiln axle. After insertion of this axle it is sealed with a suitable refractory compound which will .permit said axle to rotate within said compound while acting as a seal for the combustion gases.

Preferably said burners are supplied with an excess of secondary combustion air for producing a slight positive pressure within the furnace as compared to atmospheric pressure. thus assisting the draft of said furnace and helping to maintain uniformity of temperature within said furnace.

A furnace thus designed can be kept at a fairly even temperature within very narrow limits for long periods of time.

The rotary multitubular kiln of the invention is shown in vertical cross section in FIG. 1, as seen from the front end of the furnace structure, while FIG. 2 shows this kiln in side view. In the embodiment shown, the kiln has nine process tubes with an approximate internal diameter of 250 mm. each and a length of 2,800 mm. The tubes are arranged in a circle with an approximate radius of 450mm. center to center with the tubes evenly distributed on this circle. It will be understood that the diameter of the tubes and their total length will vary within certain limits, but the number of tubes forming the cage is not so restricted, for it is feasible, by increasing the overall diameter of the cage, to accommodate more tubes, which may even be disposed in two or more concentric circles, as will be un; derstood by those conversant with the art. Said tubes have, as stated, an internal diameter between about 200 and 300 mm. A greater diameter implies that a greater mass of material under treatment would have less proportional surface area exposed to the 'action of the air or gas used for roasting. On the other hand, too small a diameter, less than about 200 mm., would interfere with easy loading and unloading of said tubes.

The rotary multitubular kiln consists of a hollow axle 30, two shallow disclike end boxes 32 and 34, and tubes 36 which pass through the external end box 32 and have open ends. The external box 32 is filled with a suitable refractory compound. The internal end box 34 is merely abutted by the tubes. Between boxes 32 and 34, one or more stiffening spiders may be included in order to strengthen the structure; they are not shown in the drawing, but those conversant with the art will understand their object.

Outside of the external box 32, each tube is closed by a suitable door 30 provided with suitable closing studs and wing nuts as at 40; the outer door contains a central port- 42, open to atmosphere and serves both for supplying reaction air to the tubes and as a peep sight for observing the progress of the roasting process.

The internal end box 34 which is abutted by the tubes 36 on its inner face, is fastened to axle 30 by means of bushes 37, but the hollow axle is formed with a series of perforations in the zone between the faces of said box, as indicated at 44. A central perforation 48 is provided for each tube on the abutting wall of box 34, and thus said tubes appear to be partially closed by a diaphragm 46. Perforation 48 is for the exit of the process gases arising from the roasting of the sulfide. The gases converge towards the axle 30 at the perforations 34 and are thence evacuated through the end of the hollow axle beyond the rear wall of the furnace, as will be explained in due course.

The rotary multitubular kiln is mounted on a truck 50, for ease of erection and for carrying out future inspection, maintenance and repairs. This truck consists of a platform 52 covered with fire brick and fits easily between the well walls of the furnace. Both the sidewalls of said well and the edges of the truck may be provided with channel sections which intermesh when the truck is in working position, and loose sand is placed in these intermeshing sectionsto provide a sand seal for the combination gases, thus impeding escape of combustion gases through the underpart of said truck towards the front region of the furnace (see 51 in FIG. 1).

As the truck is destined merely for occasional travel, it does not appear practical to provide it with wheels and axles with bearings that require lubrication, as they are exposed nearly permanently to heat from the furnace. Hence it has been preferred, for practical reasons, to provide the platform of this truck 50 with a pair of inverted flat-bottomed rails on each side of the platform, parallel to each other'and to the longitudinal center plane of the truckand furnace.

in the well of the furnace and extending out beyond the front face of the furnace and atthis same level, two other pairs of such rails are provided. Additionally, in an upright position, and thus jointly in section, as indicated at 53, four cooperating rails are provided to form a compounded ball race. Steel balls 53, of say 70- 100mm. diameter, are; provided, interspaced as desired along said tracks, and thus it becomes possible to move the truck 50 with its built-on rotary kiln with comparative ease, to run it in or out of the furnace when it becomes necessary. This gives a simple and efficient, relatively economical, solution for occasional movements of this truck and supported structure.

The rotary multitubular kiln is assembled on this truck and the kiln axle is supported by a convenient bearing 56 mounted on that end of the truck that will remain outside the furnace when it is in working position. This'bearing is preferably adjustable in height by means of liners or other suitable means.

for the good performance of the kiln during roasting. Should the tubes have a slight inclination toward one of their ends,

This is necessary, as horizontally of the system is rather critical notwithstanding the fact that they are all parallel to each other would tend to migrate towards the lower ends thereof and interfere with the processQl-lowever, should material accumulate at one end during a long period of work, this is easily corrected by the use of a long handled rake having a semicircular plate mounted normally'of said handle-The kiln attendant, by inspection through the peep holes 42, can easily observe when it is necessary to redistribute the mass of material within the tubes and can carry out this task expeditiously by opening the corresponding door 38, without having to stop the rotation of the kiln, as said rotation is slow. FIG. 4 shows a useful form of implement for this task. I

The other or rear end of the axle of the kiln is supported,

I once the kiln has been brought to its position in the furnace,

by a bearing located outside and'behind the rear endof the furnace, as at 60. Bearing 60 should also be adjustable in height, in the manner already explained for supporting bearing 58 located at the forward end of this axle. During construction or erection of the kiln, and later, while removing or replacing the kiln carrying truck 50 from or to the furnace, for purposes of inspection or maintenance, axle 30 is not supported by the rear bearing, but the inner boxlike structure 34 will rest on suitable supports 49 provided on the after part of truck 50. These supports are dimensioned to give this end' of the multitubular kiln a vertical play of about one-half inch after removal of the bearing 60 or before placing the truck inside the furnace, and will thus provide a'suitable support for the kiln when required or during displacement. Proceeding as stated, after the truck with the kiln are brought inside the furnace, the kiln can be leveled, as by jacking, in order to avoid as far as possible migration of material being processed in said kiln.

and to the supporting axle, the materialwithin said tubes treatment. Both purposes are achieved by the following means:

Firstly, the rear end of hollow axle 30, behind the rear wall of the furnace at 61, is conically restricted and introduced into an ejector box 62 by the use of a suitable bearing 64 which forms a seal resistant to the hot gases flowing through. Ejector box 64 is supplied with cold compressed air at say 30 p.s.i.; the air and entrained gases leave the ejector box 64 and are passed through a bank of cyclones destined for the recuperation of the entrained fines and then to atmosphere through another flue or chimney (not shown), and if necessary a suitable fan 81 is interposed between said bank of cyclones and said flue (not shown), which helps circulation and extraction of the process gases and evacuates them to atmosphere. Thus a partial vacuum is produced in both the hollow axle 30 and the tubes 36, allowing atmospheric air to enter tubes 36 through the peep holes 42, pass through tubes 36 and exit therefrom by ports 48 to inside the hollow box 34 and, following through the perforations 44 in said axle, continue through the hollow axle 30 towards the ejector box 62, and, hence, to the cyclones and the corresponding flue. At the same time air. is drawn through the end 31 of the hollow axle 30, but this air, due to its rapid and unobstructed path through axle 30, does not undergosufficiently rapid heating to raise its temperature to the working temperature of the furnace, and this air helps to cool the process gases before the cold air within injector box 62 finally brings them down to a reasonable temperature for the gases to pass to the cyclones, around 250--270 C. .being convenient for the operation of recovering fines for recycling to the kiln, as will be understood. it is convenient to provide suitable valve means on the end 31 of axle 30, for better control of air passing throughsaid axle and, at the same time, this also controls passage of air into tubes 36 through the peep holes 42. V I

lf desired, axle 30 between the endboxes 32 and 34 may be coated with fireclay or other heat insulation to keep down the temperature of the air passing through said axle. It is also convenient to incorporate a fixed spiral deflector plate within the end box 34 which would conduct any fines to the perforations .44 in axle 30, avoiding fines accumulating in said box 34, as

will be understood. This is not shown in the drawings.

Should a battery of such kilns as herein described be installed it might become economically feasible to treat the process gases, which are separate from-the combustion gases, for recuperation of the S0 given off during roasting of the sulfide, for use in making sulfuric acid or in any other conventional manner.

It should be stressed that, during working of the kiln, the front disclike box 32 lying outside the furnace, also acts as a rotary door for said furnace. Therefore, the inner face 320 of this box rests against the front wall 22 of the furnace, wall 22 being built up after the kiln bearing-truck is placed inside the furnace. A suitable smooth ring surface is prepared on the out side of this wall, against which the edge portions of this disclike end box 32 abut with interposition of a suitable asbestos base gasket 33 which provides a suitable seal for the combustion gases within the furnace under a slight positive pressure, as hereinbefore mentioned.

The end wall 22 of the furnace, built after the kiln on its truck is in place, must be pulled down when it becomes necessary to remove the truck and kiln from thefurnace, for inspection or maintenance purposes. To facilitate this operation, wall 22 should be designed in such a manner that these operations can be carried out expeditiously. After inspection or maintenance has been carried out, the truck with its supported kiln is again brought into the furnace, leveled and wall 22 is rebuilt.

In operation, each tube is charged at predetermined time intervals as uniformly as possible with a molybdenite or other sulfide concentrate. The operation is facilitated by the use of suitable means, as for example a long semicircular channelshaped spade made of sheet metal (FIG. 3). The diameter of this implement should be suitable for the diameter of the tubes 36. The concentrate is previously placed on this implement, evenly spread, and the whole is inserted inside tubes 36 and, by a simple 180 turn about the longitudinal axis of this channel, the molybdenite or other concentrate is suitably distributed inside tubes 36. This operationis easily and quickly done by two men, for the weight of a normal charge for each tube rarely will be over 120 pounds, including the weight of said implement. The produced oxides can be removed from each tube after roasting is terminated for a particular. tube, by means of the rake already mentioned (FIG. 4). The discharged product is received in a suitable movable hopper or barrow (not shown), placed when required near the end of the tube being unloaded. This task is also fairly simple and does not entail more than about 5 minutes per tube, and it may even be accomplished without stopping the rotation of the kiln. As soon as a tube has been emptied of its oxide product, it is recharged with fresh sulfide and the process continues normally. 4

Following charging of a tube with fresh sulfide, as the tube slowly rotates with the cage, the loose sulfide is partly drawn up laterally by said rotation of the tube, untilthe surface of the charge acquires its angle of repose which, for molybdenite, is between 30 and 35. In FIG. 3, we see a tube loaded with a charge corresponding to about one-fifth of the tube diameter, just after the load has been deposited inside the tube, and in FIG. 6 we see the position of said. loadwhile the tube is rotating about an axis parallel to said tube. Having reached this position of FIG. 6, the outer or top surface of. the molybdenite starts to slide down as at a, while that part of the molybdenite at b, in direct contact with the inner wall of the tube which has received the full impact of the furnace heat on the outside of tubes 36, begins to replace the downwardly flowing molybdenite at point c. In this mannenfresh hot material is exposed to the air or gases circulating inside tubes 36 and therefore a good roasting action is obtained. This renewal of the molybdenite around point goes on continuously during the roasting process in a particular tubev 36 and, as the process advances and the molybdenum trioxide content increases, the whole mass becomes properly heated; and well mixed. Thus the process continues with efficiency until. the roasting step has been fully completed, for theparticular batch contained in a given tube 36. Note that at no time or place have the combustion gases come into contactwith the material treated or with the air supplied for roasting the sulfide; thus the presence of sulfur or other contaminants-in the combustion gases can not contaminate the produced trioxide and impede the production of a molybdenum, trioxide which complies with given specifications-as to purity or quality.

Likewise the process gases never come in contact with the combustion gases. The process gases have a fairly high percentage of S0 and in practicewe have seen that the charge of a particular tube, during roasting which requires from 8 to 9 hours, gives off S0 in divers concentrations. The maximum concentration takes place roughly between-2 and 4v hours from the beginning of roasting. and is as high as 8 percent of the process gases evolved. But as the plurality of the tubes in the kiln are unloaded and loaded successively and at equally spaced times resulting from dividing said period of 8-9 hours by the number of tubes 36 in a given kiln, the result is that the total SO, concentration from said plurality of tubes stabilizes around percent to 6 percent of the process gases obtained, and this makes it feasible to treat such' gases rich in S0 when the installed sulfide roasting capacity of a plant warrants such recuperation. Such recuperation of S0 is not a part of this invention and is merely mentioned as an inherent possiblity of the system, with its semicontinuous manner of working.

Preference has herein been given to atmospheric air for roasting, but it is not difficult to appreciate that oxygen enriched air, or even industrial oxygen alone, could be supplied to tubes 36 for improving control and speeding up the roasting process. Therefore, means can be devised for injecting such gases in a suitable manner which might consist of a contact box with adequate ports for the passage of said gases into the disclike front box 32 and hence to the tubes 36 through suitable conduits. This is not shown in the drawings but those conversant with the art will understand how to carry this out without impeding the loading and unloading of the tubes. Care would have to be taken to replace the from open ports 42 by transparent, heat-resisting, sighting means for watching the progress of roasting while impeding escape of such supplied gases at the outer end of tubes, as well as means for preventing powdered sulfide from clogging the conduits for said gases.

The gases to be injected could also be reducing gases, such as hydrogen, as will be understood, when it is desired to transform molybdenum trioxide,- or other oxides that have been produced, into other molybdenum products, for instance. This is feasible in the same rotary kiln of this invention, when it is thus equipped with means for supplying reducing gases, and the product first obtained can then receive a second kiln treatment with appropriate gases, as desired.

The invention is not restricted to the particular embodiment shown in the drawings and changes may be made therein which will be understood to come within the spirit and scope of the invention, as defined by the following claims.

We claim:

1. A rotary multitubular kiln comprising:

an enclosure;

heating means within said enclosure;

an axle extending through said enclosure and mounted for rotation about a horizontal axis, said axle having a hollow portion in communication with the exterior of said enclosure;

a supporting plate carried by said axle and closing one wall of said enclosure;

a plurality of tubes substantially parallel to said axle and connected to rotate with said axle;

one end of each tube extending through said supporting plate to the exterior of said enclosure and being adapted to permit the direct introduction of material to be roasted thereinto and the direct removal of said material therefrom; and

means for conducting gas from the other end of each tube to said hollow part of said axle, while inhibiting the movement of said material to be roasted from said tubes into said hollow part.

2. A kiln according to claim 1, in which the tubes have an internal diameter in range of 200 to 300 millimeters, and a length in the range of 2.50 to 3.00 meters.

3. A kiln according to claim 1, in which the axle is hollow throughout its length, and means are provided for controlling the flow of gas through the axle.

4. A kiln as claimed in claim 1 in which said tubes are provided with openings positioned outside said enclosure to admit ambient air thereinto and comprising extractor means connected to draw gas from the hollow part of said axle, thereby drawing gas intothe axle from the tubes and ambient air into said tubes.

5. A kiln according to claim 1, in which the axle is mounted on a movable truck, supported by a system of steel balls that travel over suitable guides placed both under said truck and the kiln floor of the furnace.

6. A kiln according to claim 1 comprising an extractor mounted. at the end of the axle of the kiln to withdraw gases from the kiln, a cyclone mounted to receive gases from the extractor, and a further extractor mounted beyond the cyclone.

7. A combination according to claim 1 in which the furnace is heated by electrical resistance heaters.

8. A combination according to claim 1 in which said supporting plate forms part of a box filled with refractory material and the inner surface of the box is in contact with the outer surface of the enclosure with an intermediate heat resistant gasket so as to maintain a gastight seal on rotation of the kiln in the enclosure.

9. A combination according to claim 2 in which the enclosure is a double combustion chamber reverberatory furnace, and said axle is supported in bearings both of which are vertically adjustable outside the walls of the furnace.

10. A combination according to claim 9 in which the double combustion chambered reverberatory furnace is heated by not less than four burners for liquid fuel and the burners are fed with an excess of combustion air in order to produce a slight positive pressure within the furnace.

11. A combination according to claim 10, in which one or more pyrometers are provided for measuring the working temperature within the furnace.

12. A combination according to claim 10 in which one or more pyrometers are provided for measuring the working temperature within the furnace, and the burners adjusted by automatic control using the signals from the pyrometers. 

2. A kiln according to claim 1, in which the tubes have an internal diameter in range of 200 to 300 millimeters, and a length in the range of 2.50 to 3.00 meters.
 3. A kiln according to claim 1, in which the axle is hollow throughout its length, and means are provided for controlling the flow of gas through the axle.
 4. A kiln as claimed in claim 1 in which said tubes are provided with openings positioned outside said enclosure to admit ambient air thereinto and comprising extractor means connected to draw gas from the hollow part of said axle, thereby drawing gas into the axle from the tubes and ambient air into said tubes.
 5. A kiln according to claim 1, in which the axle is mounted on a movable truck, supported by a system of steel balls that travel over suitable guides placed both under said truck and the kiln floor of the furnace.
 6. A kiln according to claim 1 comprising an extractor mounted at the end of the axle of the kiln to withdraw gases from the kiln, a cyclone mounted to receive gases from the extractor, and a further extractor mounted beyond the cyclone.
 7. A combination according to claim 1 in which the furnace is heated by electrical resistance heaters.
 8. A combination according to claim 1 in which said supporting plate forms part of a box filled with refractory material and the inner surface of the box is in contact with the outer surface of the enclosure with an intermediate heat resistant gasket so as to maintain a gastight seal on rotation of the kiln in the enclosure.
 9. A combination according to claim 2 in which the enclosure is a double combustion chamber reverberatory furnace, and said axle is supported in bearings both of which are vertically adjustable outside the walls of the furnace.
 10. A combination according to claim 9 in which the double combustion chambered reverberatory furnace is heated by not less than four burners for liquid fuel and the burners are fed with an excess of combustion air in order to produce a slight positive pressure within the furnace.
 11. A combination according to claim 10, in which one or more pyrometers are provided for measuring the working temperature within the furnace.
 12. A combination according to claim 10 in which one or more pyrometers are provided for measuring the working temperature within the furnace, and the burners adjusted by automatic control using the signals from the pyrometers. 